It has been known in the past to arrange a catalyst able to store oxygen in an exhaust passage of an internal combustion engine and to remove unburned gas (HC, CO, etc.) and NOX in the exhaust gas at the catalyst. The higher the oxygen storage ability of the catalyst, the greater the amount of oxygen able to be stored at the catalyst and the more the exhaust purification performance of the catalyst is improved.
To maintain the oxygen storage ability of the catalyst, it is preferable to make the oxygen storage amount of the catalyst fluctuate so that the oxygen storage amount of the catalyst is maintained constant. In the internal combustion engine described in PTL 1, to make the oxygen storage amount of the catalyst fluctuate, the target air-fuel ratio of the exhaust gas flowing into the catalyst is alternately switched between a lean air-fuel ratio leaner than a stoichiometric air-fuel ratio and a rich air-fuel ratio richer than a stoichiometric air-fuel ratio based on an output of a downstream side air-fuel ratio sensor arranged at a downstream side of the catalyst in the direction of flow of exhaust.
Specifically, when the air-fuel ratio detected by the downstream side air-fuel ratio sensor becomes a rich judged air-fuel ratio richer than a stoichiometric air-fuel ratio or becomes less, the target air-fuel ratio is set to the lean air-fuel ratio, while, when the air-fuel ratio detected by the downstream side air-fuel ratio sensor becomes a lean judged air-fuel ratio leaner than a stoichiometric air-fuel ratio or becomes more, the target air-fuel ratio is set to the rich air-fuel ratio.